Add note section for removing line that won't work in Rails 4

With the `attr_accessible` line in it gives this error:

    $ bundle exec rspec
       c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activemodel-4.0.0/lib/activ
e_model/deprecated_mass_assignment_security.rb:14:in `attr_accessible': `attr_ac
cessible` is extracted out of Rails into a gem. Please use new recommended prote
ction model for params(strong_parameters) or add `protected_attributes` to your
Gemfile to use old one. (RuntimeError)
        from c:/Users/blake/code/contact_manager_2/app/models/person.rb:2:in `<c
lass:Person>'
        from c:/Users/blake/code/contact_manager_2/app/models/person.rb:1:in `<t
op (required)>'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:228:in `require'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:228:in `block in require'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:213:in `load_dependency'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:228:in `require'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:329:in `require_or_load'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:462:in `load_missing_constant'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:183:in `const_missing'
        from c:/Users/blake/code/contact_manager_2/spec/models/person_spec.rb:3:
in `<top (required)>'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:222:in `load'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:222:in `block in load'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:213:in `load_dependency'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activesupport-
4.0.0/lib/active_support/dependencies.rb:222:in `load'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rspec-core-2.1
4.5/lib/rspec/core/configuration.rb:896:in `block in load_spec_files'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rspec-core-2.1
4.5/lib/rspec/core/configuration.rb:896:in `each'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rspec-core-2.1
4.5/lib/rspec/core/configuration.rb:896:in `load_spec_files'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rspec-core-2.1
4.5/lib/rspec/core/command_line.rb:22:in `run'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rspec-core-2.1
4.5/lib/rspec/core/runner.rb:80:in `run'
        from c:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rspec-core-2.1
4.5/lib/rspec/core/runner.rb:17:in `block in autorun'

I believe the attr_accessible stuff is taken care of in the controller in Rails 4 so we might need to also update the controller to account for this.

Not sure if you want these Rails 4 specific changes in this tutorial or not?